The present invention relates, in general, to offshore technology and, in particular, to a new and useful method that makes cylindrical piling easier to drive. The method works particularly well for piling driven in sensitive clays. In addition to making the piling easier to drive, the installation operations required to install piling for an offshore platform are greatly facilitated. The method involves putting a plug in the pile at or near its bottom end and leaving this plug in place during the driving of the pile.
The use of piling in offshore platforms has evolved over many years and includes a wide variety of known operations. Generally, however, the pile is driven either through the legs of a jacket or template already in place on the sea floor or through sleeves attached to the jacket.
In any event, a derrick barge carrying lengths of pile as well as pile add-ons is generally secured adjacent the site where the driving operation is to occur. These pile add-ons extend the length of the pile until it reaches the seabed. This pile is then driven into the seabed with additional add-ons being supplied as needed until the desired depth is reached.
Occasionally, removable closure plates are installed in the pile to add buoyancy to selected sections of the pile. This buoyancy reduces the hook load on the derrick that must be provided to manipulate the pile and its add-ons as it is lowered to the seabed.
U.S. Pat. Nos. 4,696,603, 4,696,604, and 4,705,430 disclose various compliant tower or composite leg platform designs. These structures require piling that is much longer and heavier than that which has been installed to date. A single pile for a compliant tower may weigh 2,000 tons and have a length of over 2,000 feet. In contrast, a current single skirt pile might weigh only about 400 tons and have a length of about 550 feet. Thus, currently known problems regarding the manipulation and lowering of existing piles with add-ons are greatly exacerbated by the 4 or 5 fold increase in pile weight and length associated with such compliant towers.
U.S. Pat. No. 5,060,731 discloses a well conductor, as contrasted with a pile, that is plugged and then driven to a design penetration depth afterwhich the plug is drilled out as the drilling rig begins drilling the well. However, as can be appreciated, conductors are normally 22 to 30 inches in diameter whereas piling for offshore platform foundations is often in the 72 to 96 inch diameter range. Also, such well conductors are often only driven to a rather shallow range of about 200-300 feet as compared to the driven depth for anchoring purposes which involves considerably deeper penetrations.
It is, therefore, an object of this invention to make piling easier to drive regardless of its length, desired penetration depth, or diameter. A further object of this invention is to reduce or eliminate the large support structures on the platform that are normally required or associated with the driving of piles. Yet another object of this invention is to facilitate and expedite pile driving operations thereby reducing their costs. It is also an object of this invention to achieve all of the operational advantages of a sealed pile, such as its control and buoyancy advantages, without the disadvantage of having to remove any of the seals. Another object of this invention is to make piles easier to drive in sensitive clays. Driving may also be easier in other types of soils, but the effect will be less pronounced than in sensitive clays. Still another object of this invention is to make the pile easier to transport, handle at the site, assemble, and lower.
For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which the preferred embodiments of the invention are illustrated.